Oxygen Transport by Diffusion

Gas transport within the majority of emergent and submerged aquatic plants is believed to be driven primarily by diffusion rather than by convective flow (Armstrong, 1979; Sorrell and Dromgoole, 1987; Larkum et al., 1989). Passive gas phase diffusion within seagrasses occurs continuously along the downhill partial pressure gradients from leaves to rhizomes to roots. In the light, the high

0 25

Fig. 8. Changes in oxygen partial pressure in meristematic tissues of an intact eelgrass shoot during a light-dark transition experiment. The rapid (~45 min) establishment of a low steady state oxygen partial pressure in the dark strongly suggests that rates of oxygen transport between tissues and losses to respiration and external media are high (Redrawn from Greve et al., 2003).

0 25

50 75 100 Time (min)

125 150

Fig. 8. Changes in oxygen partial pressure in meristematic tissues of an intact eelgrass shoot during a light-dark transition experiment. The rapid (~45 min) establishment of a low steady state oxygen partial pressure in the dark strongly suggests that rates of oxygen transport between tissues and losses to respiration and external media are high (Redrawn from Greve et al., 2003).

oxygen partial pressure in leaves generated by photosynthesis create steep gradients from leaves to water column and less steep gradients from leaves to below-ground tissues. During darkness, the oxygen partial pressure in leaves declines below that of the water column, and the oxygen flux becomes directed from water to leaf, instead of vice versa (Greve et al., 2003). Although weaker than in the light, the gradient from leaf to rhizomes and roots persists during darkness ensuring a continuous supply to below-ground tissues.

Rapid changes in the oxygen content of meris-tematic tissues in the transition between leaves and rhizomes of eelgrass suggest that rates of internal oxygen transport and losses to the external media are high and that internal oxygen pools are relatively short-lived (Fig. 8; Greve et al., 2003). A thorough examination of oxygen losses in the submerged freshwater macrophyte Egeria densa (Sorrell and Dromgoole, 1987, 1988) showed that internal pools of oxygen were depleted rather slowly (up to 4 h) probably due to high resistance toward gas exchange between leaves and water column. Seagrasses may have more gas permeable leaves because time intervals between the occurrence of new steady state oxygen balances in both Zostera marina (Fig. 9; Greve et al., 2003) and Cymodocea rotundata (Pedersen et al., 1998) were less than 2 h after light-dark switches. One consequence of this apparently high permeability is that the internal pool of oxygen built up by photosynthesis during the day is insufficient to support night-time respiration of leaves, rhizomes and roots, in contrast to what is often supposed (e.g. Smith et al., 1984; Touchette and Burkholder, 2000).

Rapid internal transport of oxygen by passive diffusion from the leaves of Zostera marina to the meristematic region and further on to rhizome in-ternodes is also demonstrated by changes in internal oxygen partial pressures after manipulation of water column oxygen concentrations during darkness (Fig. 9A). Water column oxygen was lowered stepwise from atmospheric equilibrium to zero, and after each step, new steady-state oxygen partial pressures were rapidly attained within the meristematic tissue and at two positions along the rhizome. An oxygen gradient persisted throughout the experiment with the highest oxygen partial pressure in the meristematic region and the lowest in the oldest rhizome internode. At a water column oxygen partial pressure corresponding to about 25% of air saturation the most distal rhizome internode became close to anoxic, but traces of oxygen were still observed in rhizome internode #3 and in the meristematic tissue reflecting a continuous transport of oxygen by passive diffusion.

The experiment with stepwise reduction in water column oxygen concentrations makes it possible to estimate the velocity of internal oxygen transport within the rhizome of Zostera marina (Fig. 9B). For each step, there was a consistent lag period between the time when water column oxygen had started to decline until changes in the oxygen partial pressures within the rhizome sections were recorded. The distance between the meristematic region and rhizome internode #4 was about 5 cm, and the traveling time for oxygen over that distance was 4-5 min clearly reflecting rapid gas phase diffusion.

C. Oxygen Transport by Mass Flow

Mass flow of lacunal gasses has been demonstrated for several emergent plants (Dacey, 1981; Armstrong and Armstrong, 1990; Brix et al., 1992), but major oxygen transport by mass flow likely requires through-flow provided by tissue contact with the atmosphere. In submerged plants, mass flow could theoretically occur on a small scale driven by internal pressurization generated from photosynthesis or by leaf movement due to waves or water current. However, gas phase diffusion should be sufficient to ensure oxygen transport in submerged plants as

Was this article helpful?

0 0

Post a comment